In 1977, 1,950 adult residents of Washington County, MD were examined for respiratory symptoms and impaired ventilatory function by means of standardized procedures. Nearly half of these persons had never smoked; the remainder were cigarette smokers. By matching records from these respiratory examinations against the records from a private census of the county in 1975, it will be possible to identify which of the examined subjects were exposed to smokers in the household in 1975. This will give a close approximation of the 1977 exposure because relatively few persons changed their smoking habits during this two-year period. Other household exposures of possible importance for respiratory findings are respiratory infections in the household, oxides of nitrogen, and summer pollens. The effects of these other exposures can be assessed by indirect indices: Probability of respiratory infection by the number of school children in the household; oxides of nitrogen by the use of gas for heating or cooking; and summer pollens by location of residence and the absence of air conditioning. The analysis will compare the frequency of respiratory symptoms and impaired ventilatory function among smokers and nonsmokers with and without these exposures, using appropriate statistical adjustments for the effects of other exposures and confounding factors such as race, sex, age and socioeconomic status.